The Centers for Disease Control and Prevention says it doesn’t know what’s causing a sudden rise in cases of a frightening, polio-like condition that leaves children paralyzed or with weakened limbs.

The No. 1 suspect had been a virus called enterovirus D68, or EV-D68. In 2014, a wave of cases of acute flaccid myelitis coincided with outbreaks of EV-D68 across the country.

But the CDC says it has not consistently found EV-D68 in confirmed cases since then. Officials say they’re looking at a range of possible causes.

However, doctors who have been studying children affected by acute flaccid myelitis say they have gathered a growing body of evidence that EV-D68 is the main cause, and that the virus may have changed in recent years in ways that make the paralyzing side-effects more likely.

They’ve documented an increase in cases of EV-D68 surrounding outbreaks of acute flaccid myelitis. Experiments have also shown that EV-D68 can invade nerve tissue, including the spine, and there’s also evidence of genetic changes in the virus itself.